A field effect transistor (FET) has been widely used as a unit electronic device (element) for a semiconductor memory integrated circuit, a high-frequency signal amplifier device, a liquid crystal driving device, and the like. A thin film transistor (TFT) is classified as the field effect transistor. In recent years, the TFT has been widely used as a switching device for various displays that have been rapidly developed. Examples of such displays include a liquid crystal display (LCD), an organic electroluminescence (EL) display, and the like.
The LCD has been the mainstream in the fields of medium and small-sized display panels and large-sized image display panels used for TV applications. Since the organic EL display can achieve high resolution as compared with the LCD, future development thereof has been desired.
The frame rate of the LCD has been increased along with an improvement in moving picture resolution and the spread of three-dimensional displays. It is effective to drive the LCD at a high frame rate for improving the moving picture resolution, and a further increase in high frame has been desired. A large screen, a high resolution, and high frame rate drive have been the keys to the development of displays, and the TFT has been required to achieve performance necessary for implementing these requirements. For example, the TFT has been required to achieve high mobility along with an increase in pixel capacitance for implementing a large screen, an increase in the number of scan lines for implementing a high resolution, and an increase in frame rate.
The mobility of an a-Si:H (hydrogenated amorphous silicon) TFT used for the LCD is about 2 cm2/Vs or less. However, it has become difficult to deal with the above requirements (i.e., large screen, high resolution, and high frame rate drive) for displays when the mobility of the TFT is about 2 cm2/Vs or less.
Since the organic EL device is a current-driven device, and an increase in current value of a drive TFT is required to improve the luminance of the screen, a high-mobility TFT is indispensable for the organic EL display. The TFT used to drive the organic EL display is also required to exhibit current stress reliability in addition to high mobility. Low-temperature poly-Si (LTPS) has been considered to be a candidate for a TFT material that satisfies both high mobility and current stress reliability. When using LTPS, however, the screen size that can be implemented may be limited due to the beam length used during laser crystallization, and the TFT characteristics may show in-plane non-uniformity due to an inter-shot variation of laser light.
A TFT that utilizes an oxide semiconductor instead of a-Si:HTFT or LTPS has attracted attention. For example, a TFT in which an oxide semiconductor such as zinc oxide (ZnO) or indium gallium zinc oxide (IGZO) is used for an active layer (semiconductor layer) exhibits high mobility, and development thereof has progressed.
The oxide semiconductor includes a highly ionic bond, and is characterized in that the difference in electron mobility is small between a crystalline state and an amorphous state. Specifically, a relatively high electron mobility can be implemented even in an amorphous state. Moreover, since the oxide semiconductor is rarely affected by the grain boundary barrier even when the oxide semiconductor is crystallized, it is possible to produce a TFT that is suitable for an increase in area that requires in-plane uniformity. Since a gap state derived from oxygen deficiencies is present in the vicinity of the valence band, holes do not easily become a free carrier as compared with electrons. Therefore, the off current during the operation of the TFT can be reduced to about 10 to 15 A. Since the oxide semiconductor is a wide-bandgap semiconductor as compared with a silicon-based material (TFT), the oxide semiconductor exhibits excellent stability to light in the visible region. Since an amorphous oxide semiconductor film can be formed at room temperature by utilizing a sputtering method or the like, studies have been conducted to form an oxide semiconductor film transistor on a resin substrate (e.g., PET).
As a TFT technique that utilizes the oxide semiconductor, Patent Document 1 discloses a semiconductor device that utilizes a crystalline oxide having an electron carrier concentration of less than 2×1017/cm3 as an n-type semiconductor, and exhibits excellent stability, uniformity, reproducibility, heat resistance, durability, and the like.
Patent Document 2 discloses a TFT in which indium tin oxide (ITO) or the like is used for the channel layer as an oxide conductive material having a high carrier concentration. In Patent Document 2, a reduction in leakage current and an improvement in subthreshold factor are achieved by forming the channel layer consisting of an very thin film (6 to 10 nm) to have a uniform thickness, and planarizing the surface of the gate insulating film to improve the interfacial characteristics.
In Patent Document 3, an oxide semiconductor film having a carrier concentration of about 1×1018 cm−3 is formed by sputtering an oxide sintered body in which gallium is solid-dissolved in indium oxide.
In Patent Document 4, the on-off ratio of a bottom-gate TFT is improved by increasing the oxygen density in the surface layer of the oxide semiconductor as compared with the side of the gate insulating film by applying oxygen-containing plasma.
Patent Document 5 discloses an oxide TFT that includes two active layers formed of indium zinc oxide (or ITO) and GIZO, and states that high mobility and a good threshold voltage are obtained. Specifically, the TFT disclosed in Patent Document 5 has a configuration in which a GIZO layer having a thickness of 60 nm and a low carrier concentration is formed on an indium zinc oxide (or ITO) layer having a thickness of 5 nm and a high carrier concentration.
However, the techniques disclosed in Patent Documents 1 to 5 have the following problems.
According to the technique disclosed in Patent Document 1, since the electron carrier concentration is less than 2×1017 cm−3, it is necessary to improve the mobility.
According to the technique disclosed in Patent Document 2, since the thickness of the channel layer is 10 nm or less, the channel layer may be formed in the shape of an island, and an area in which the semiconductor layer is not formed may occur in the channel layer.
According to the technique disclosed in Patent Document 3, since regions that differ in carrier concentration are not provided in the oxide semiconductor layer, an improvement in subthreshold factor is required.
According to the technique disclosed in Patent Document 4, a high oxygen density region is formed in the semiconductor layer by oxygen plasma treatment to improve the ON/OFF ratio. However, high mobility is not obtained.
According to the technique disclosed in Patent Document 5, since the channel layer having a two-layer structure is necessary, a decrease in productivity and an increase in production cost occur as compared with the case of forming the channel layer using a single material.